“We’re still talking about rare trends,” says James Alan Fox, a criminologist at Northeastern University and author of the book, “Violence and Security on Campus.” “Contrary to people’s viewpoint, there’s no increase.... With almost 20 million college students in the US, and almost 20 homicides per year [on college campuses], on average, the probability is literally one in a million.”

According to data compiled by the US Department of Education and the Safe Schools Initiative, the campus homicide rate is about 1 death per one million people, compared with 57 deaths per million in the general population. (Campus homicide rates include murder by other means, in addition to campus shootings.)

Statistics on campus shootings are scarce and often conflicting, partly due to varying definitions of what constitutes a campus shooting. Some reports do not count shootings if there were no fatalities while others do not include incidents that occurred off-campus. However, most data suggest there are between one and three fatal campus shootings per year across the US.

An analysis by Slate and Jessie Klein, author of “The Bully Society,” found 39 college shootings between 1980 and 2012, in which at least one person other than the shooter died.